Mercedes D.II

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The Mercedes D.II was a 6-cylinder, liquid cooled inline aircraft engine built by Daimler during the early stages of World War I. Producing about 110 to 120 hp, it was at the low-end of the power range of contemporary engines, and was generally outperformed by rotaries who's power-to-weight ratio tended to be much better. It also had stiff competition from the Ferdinand Porsche-designed 120 hp Austro-Daimler. The D.II was produced only briefly as a result, but its design formed the basis for the later Mercedes D.III which saw widespread use throughout the war.

The D.II was based on the Austro-Daimler to a large degree. Like the Austro-Daimler, it was built up from the crankcase, which was milled from two pieces of cast aluminum bolted together at their midline. The cylinders were separately milled from steel and bolted to the top of the crankcase. Steel sleeves were fitted over the cylinders and welded on to form a cooling jacket. Much of this complexity is due to the differential rates of expansion of steel and aluminum, which precluded screwing the cylinders into the crankcase, and the alloys of the era meant that an aluminum cylinder was not possible. Both also used a scavager pump to pump oil out of the crankcase to a separate cylinder, where a second high-pressure pump supplied oil to the engine. This arrangement allowed for a much smaller "sump" on the bottom of the crankcase, reducing the overall size of the engine, although not nearly as much as in the case of the Austro-Daimler.

Where the D.II differed from the Austro-Daimler was largely in arrangement. For instance, the D.II featured a single overhead cam, powered by a shaft leading up from the crankshaft at the rear of the engine, whereas the Austro-Daimler had a more "conventional" system using pushrods driven off the crank. Another unique feature was the ability to shift the camshaft to a "half compression" position for starting. Additionally, the D.II used two carburetors located together feeding the cylinders through two pipes, whereas the Austro-Daimler separated their carburetors to locate them closer to the cylinders they fed. The D.II also used a unique cooling jacket design, with every two cylinders being covered by a single jacket.

The D.II saw some use, notably on the Fokker D.I, Halberstadt D.II, Aviatik B.II and Albatros B.1 and B.2. It was fairly quickly replaced by the D.III, and ended production around 1916. The D.III was essentially a scaled-up D.II, although it abandoned the paired cooling jackets.

[edit] Specifications

• Configuration: separate-cylinder straight-6 liquid-cooled in-line
• Displacement: 579 cu. in.
• Weight: 449 lb
• Power: 120 hp @ 1400 RPM